Long period vertical seismograph



Dec. 13, 1966 P. WILLMORE LONG PERIOD'VERTICAL SEISMOGRAPH 5 Sheets-Sheet 1 Filed June 1'7, 1963 Dec. 13, 1966 P. L. WILLMORE LONG PERIOD VERTICAL SEISMOGRAPH 3 Sheets-$heet 2 Filed June 17, 1963 a 8 Q mm k WV n m I Q a wm 8 8 x mm v Q R mm mm Wm mm mm Wm Q a w mm mm Dec. 13, 1966 P. L. WILLMORE LONG PERIOD VERTICAL SEISMOGRAPH 3 t e e h 5 S V 6 e h S 5 Filed June 17, 1963 m w mm on Mm Qw mm mm mm mm mm United States Patent 3,292,145 Patented Dec. 13, 1966 Free 3,292,145 LONG PERIOD VERTICAL SEISMOGRAPH Patrick Lever Willmore, Callington, Edinburgh, Scotland, assignor to United Kingdom Atomic Energy Authority, London, England Filed June 17, 1963, Ser. No. 288,320 Claims priority, application Great Britain, June 28, 1962, 24,87 8/ 62 4 Claims. (Cl. 340-17) This invention relates to vertical seismographs that is, to seismographs for obtaining a record of vertical movements of the earth.

In such seismographs a mass carrying beam is hinged at one end to a rigid frame and held up by a spring attached to the other end. Movement of the earth produces a corresponding movement of the rigid frame, but the mass carried by the beam does not move to the same extent because of its inertia. The relative motion of the frame and the mass can be detected and can therefore provide a record of the movement of the earth.

Instrumental development has moved away from the ideal of accurate reproduction of earth movement and has turned towards the production of seismographs having enhanced response in selected frequency ranges. One class of seismograph which has become known may be termed long period seismographs. In this class the period of natural oscillation is greater than about 2 seconds. With seismographs of the type to which this invention relates the practicable upper limit is 30 seconds. It is diificult to maintain the stability above this figure.

Early seismographs for long periods used complex spring systems and were diificult to stabilise. The first practicable long period vertical seismograph using a single spring consisted of a mass suspended on a boom which was fixed to a hinge support at one end and was held up at the other end by a coil spring in which the tension was proportional to the length of the spring, thereby necessitating the use of a spring whose relaxed length would be zero.

The weight of the single coil spring required in such seismographs is normally about 2 lbs. and the natural frequencies of free vibration are in the range about 3 to 25 cycles per second. The system, therefore, suffers from the disadvantage that the spring itself may be set into vibration by high frequency movements of the earth at or near the above mentioned natural frequencies of the spring, thereby interfering with the proper working of the seismograph.

This invention provides an improved long period vertical seismograph which is substantially unaffected by high frequency components of the earths motion.

The invention consists in a vertical seismograph which comprises a rigid frame, a mass carrying boom hinged to the rigid frame, so that the boom can be constrained to swing about a horizontal axis, a resilient connection from the frame to the beam, the resilient connection being adpted to apply to the boom a variable torque opposing the torque due to the said mass carrying boom, the said resilient connection comprising a leaf spring and a tie of substantially constant length attached at one end to the spring, the leaf spring being of such size and strength that its deflection when the beam is in its rest position is substantially equal to the length of the said link, and the said link being positioned to receive a sideways motion towards the said hinge as a consequence of a downward motion of the said mass carrying beam.

The leaf spring can, in practical systems, have a mass of only a few ounces and its natural frequency of vibration can be of the order about 400 cycles per second. More than one spring can be used if desired to support a heavy mass-carrying beam.

It is preferred that the leaf spring should be mounted as a canti-lever and should be straight when the mass earrying beam is in its rest position. In order to attain this, it appears necessary that the spring width should decrease linearly towards the tip.

The invention will be explained by reference to the drawing in which FIGURE 1 is a diagram of an embodiment in which the leaf spring is mounted as a cantilever and the tie is thin wire,

FIGURE 2 is a diagram of the same embodiment as in FIGURE 1 but redrawn to display the mathematics involved,

FIGURE 3 is an elevation of an instrument based on the embodiment of FIGURE 1 and FIGURE 4 is a plan of the instrument of FIGURE 3.

In FIGURE 1, a triangular leaf spring 1 of length L is mounted with its base fixed to a support 2 and is attached by a string 3 of length L to a weightless boom 4 typically mounted on a hinge 5. A mass 6 is held by the boom. The position of spring 1 is the equilibrium position attained supporting the boom and in this position, the spring is straight. The relaxed position of the spring is shown by the broken line 7, in this position, the spring is an arc of a circle of radius L/2.

Dotted line 8 indicates the ideal locus of the point of the triangular spring 1 to cause the mass 6 to move as if supported by a constant torque. Marks 9 indicate the position of the point of the spring for successive 10 displacement of the boom 4. Line 10 indicates the actual locus of the point of the spring 1. The ideal and actual loci are very close at the equilibrium position shown.

From the drawing the following dimensions can be recognised:

Distance betwten P and H1 is D1. Distance between P and Q is D2. Distance between Q and H2 is L. Distance between H2 and H1 is L.

If D1 equals D2 then the period of oscillation of the system should theoretically be infinitely long. Because of reactions at the various points of support, it is not possible to attain this condition but adjustment of the distance D2 with respect to D1 enables the natural period of oscillation of the system to be adjusted within the desired limits.

Some means of damping the oscillations can be used as known in the art.

In FIGURE 2 the numerals have the same significance as in FIGURE 1, mass 6 has a weight W, boom 4 has a length D H is a vertical line through P and has a length D and H H is perpendicular to H Q.

3 Let the spring have a spring constant k and its deflection at point H be y.

Then T=ky, where T is the tension in string 3. Let y=H H L' where L is an unknown length. The moment M of T about P is given by The moment M of mass 6 about P is given by M2=W.D3 sin For infinite period M +M =0 Since PH =PH =D =p=. /2 Replacing we obtain Thus if y=H H so that L -=0, and if k.D WD then M +M =0, and theoretically the period will be infinity.

In practice, an infinite period cannot be obtained since it requires perfect balancing of forces and ompletely frictionless hinges. A long period can, however, be obtained by adjusting the relative dimensions.

In FIGURES 3 and 4 a boom 4 carries a mass 6. Boom 4 is hinged to a frame 5 and is supported by two thin wires 3A and 3B attached to a cross bar 11 attached to the tips of four triangular leaf springs 1A1D, which springs are mounted on a rigid support assembly 2.

The leaf springs are straight when the boom is in its rest position, and when unstressed they were each shaped as a circular arc of radius equal to half their length.

In more detail, mass 6 consists of two heavy concentric rings 12 and 13 locked in position by grub screw 14. Boom 4 consists of a rigid tube held on a frictionless cross wire hinge assembly 15 mounted on a pedestal 5. A locking screw 16 mates with a corresponding bolt 17 and the assembly bears against shoulders 18 and 19 on tube 4 to lock the boom 4 in position. The screw 16 can be loosened to release the boom.

Wires 3A and 3B are at one end attached to a harness assembly 20 which is attached to a point on the axis of the tube 4 by a single short wire 21. At the other end wires 3A and 3B are fixed to cross bar 11. Cross bar 11 has fiat portions for receiving clamping plates 22A-22D which clamp the tips of leaf springs 1A1D to the cross bar 11.

Springs 1A-1D are clamped at their lower ends by four clamping plates 23A-23D respectively (only 23A being visible in FIGURE 4) screwed to a cranked pivot block 24 which can pivot on spindles 25 and 26. A cross plate 27 is fixed to both ends of pivot block 24 and carries a crank 28 containing a pressure pad 29 against which can bear the spindle 30 of a screw adjuster assembly 31.

Screw adjuster assembly 31 is mounted on a rigid plate 32 which is screwed to brackets 33 and 34. Flanges 35 and 36 on brackets 33 and 34 respectively enable the brackets 33 and 34 to be fixed to a heavy rigid plate 37 carrying a polished skid 38 bearing against the underside of a bridge 39 which is screwed to a base plate 40, base plate 40 being itself fixed to a rigid base 41 which carries the entire seismograph.

Rigid plate 37 has a downwardly extending centre guide 42 slidable in a channel in base plate 40. Base plate 40 supports two polished skids (only one skid 43 being visible) at opposite corners which skids bear against the underside of plate 37. A screw 44 carried in a block 45 fixed to guide 42 lies in a recess in plate 37 and bears against a face 46 of the recess.

A locking screw 47 has a spindle which locks against guide 42 when tightened and looks it in a position to facilitate transport of the seismograph.

Arms 48 and 49 are mounted on spindles 25 and 26 and carry locking screws 50 and 51 on short stub extensions (only the extension 52 on arm 48 being visible) which lock in corresponding holes in brackets 33 and 34. Arms 48 and 49 carry bars 53 and 54 against which springs 1A- 1D can bear if released.

Hinge assembly 15 is made up of thin flexible strips 55- 58 clamped at one end by plates (only plates 59 and 60 being visible) to a block 61 which has a short tubular centre section fitting round boom 4 and bolted thereto by bolt 62 and nuts 63 and 64. Block 61 has four lugs 65-69 integral therewith, to which the plates clamping the thin flexible strips 55-58 are screwed. Terminals and 71 for electrical connections are provided. Very thin wire connections between these terminals should be used to prevent interference with the swing of the boom 4.

A hollow chamber 72 is mounted on the boom 4 at one end to provide buoyancy compensation. The boom, at its other end, terminates in a platform 73 having strengthening walls 74 and 75. A micrometer 76 is carried by a holder 77 mounted on a pedestal 78. A locking screw 79 mates with a corresponding bolt 80 and, when tightened, locks the boom 4 in pedestal 78.

A bar 81 carries plungers 82 and 83 of damping coils 84 and 85, bar 81 being mounted on boom 4. Platform 73 carries a plunger 86 of a displacement transducer 87.

Rigid base 41 carries a spirit level 88 and a leveling screw assembly 89. Sockets 90 and 91 for electrical connections to the seismograph are provided,

A cover 92 can be held on the base 41 by slips 93102 and pressed down on an O-ring seal 103. Cover 92 has a window assembly 104.

An outlet in base 41 leads to a vacuum-tight union 106.

To operate the seismograph the locking screws 16 and 79 and 47 are loosened and the corresponding bolts 17 and 80 are withdrawn. The period of the thus released boom is adjusted by means of screw 44 which can be turned one way to move the entire support assembly 2 away from the hinge frame 5. When the screw is turned the other way the tension in the wires 3A and 3B exerted through the springs 1A1D is sufiicient to pull the entire assembly 2 towards the hinge frame 5. When the period reaches 30 seconds the adjustment becomes extremely critical.

Micrometer 76 enables the displacement transducer 87 to be calibrated. By adjusting screw 44 the platform 73 can be made to bear against the micrometer, which can then be turned a specified amount to provide a known displacement.

The extension of the springs 1A-1D is adjusted by means of adjuster assembly 31. When this is turned it moves spindle 30 and therefore moves crank 28. This has the effect of rotating pivot block 24 so that the extension of the springs 1A-1D is varied.

Weight 12 can be removed if desired and one or two of the springs 1A-1D disconnected to compensate for the lower mass. The effect of the damping transducers will therefore be greater.

In the embodiment shown, the point at which the wire 21 is attached to the boom is in line with the axis of the hinge and the axis of rotation of the leaf springs 1A-1D, that is, points Q, P and H in FIGURES 1 and 2 are linear when the boom is horizontal. This is a preferred arrangement but is not essential.

Other arrangements of the springs and wires are possible. For example the three points corresponding to Q, P and H can be arranged along a vertical line with the point Q uppermost, and a downwardly extending arm attached to the boom to provide the location for point H It is also within the scope 'of the invention to attach the spring to the boom and connect the tie to the rigid frame. Many arrangements can be made. In all cases however, where a cantilever is used, it is preferred that the spring be straight when the boom is in its rest position and that the tie should be tangential to the arc of movement of the tip of the spring.

Discrepancies in thickness of the spring can be compensated to some extent by adjustments in the rate at which the width of the spring changes along its length.

I claim:

1. A vertical long-period seismograph comprising a rigid frame, a mass-carrying boom hinged to said frame to allow movement of the boom about a horizontal axis, a resilient suspension for suspending the boom from the frame, said suspension comprising a cantilevermounted leaf spring connected in series with an inextensible tie, each end of said tie being secured by a fiexural connection, said leaf spring being substantially straight and said tie extending substantially at right angles from the unmounted end of the spring when the boom is in its equilibrium position.

2. A seismograph as claimed in claim 1 wherein, in the equilibrium position of the boom, said spring and tie form two substantially equilateral sides of a triangle having said hinge located substantially midway along the third side.

3. A seismograph as claimed in claim 2 wherein the leaf spring has substantially constant thickness and a width which tapers linearly from the mounted to the unmounted end.

4. A seismograph as claimed in claim 3 wherein said spring is mounted on the frame and one end of said tie is secured to a point on the boom.

References Cited by the Examiner UNITED STATES PATENTS 2,909,759 10/1959 Cook 34017 2,933,715 4/1960 Beuermann 34017 BENJAMIN A. BORCHELT, Primary Examiner.

P. A. SHANLEY, Assistant Examiner. 

1. A VERTICAL LONG-PERIOD SEISMOGRAPH COMPRISING A RIGID FRAME, A MASS-CARRYING BOOM HINGED TO SAID FRAME TO ALLOW MOVEMENT OF THE BOOM ABOUT A HORIZONTAL AXIS, A RESILIENT SUSPENSION FOR SUSPENDING THE BOOM FROM THE FRAME, SAID SUSPENSION COMPRISING A CANTILEVER-MOUNTED LEAF SPRING CONNECTED IN SERIES WITH AN INEXTENSIBLE TIE, EACH END OF SAID TIE BEING SECURED BY A FLEXURAL CONNECTION, SAID LEAF SPRING BEING SUBSTANTIALLY STRAIGHT AND SAID TIE EXTEND- 